1. Field of the Invention
The present invention relates to an image pick-up apparatus for deriving an image of an object, a fingerprint certification apparatus using the image pick-up apparatus and an image pick-up method, and more in particular, it relates to an image pick-up apparatus suitably mounted on a biological certification system such as a fingerprint certification and the like, a fingerprint certification apparatus and an image pick-up method using the image pick-up apparatus.
2. Related Background Art
In recent years, to secure security for personal information and confidential information, a biological certification system such as a fingerprint certification apparatus and the like has come to attract attention, and its demand toward office equipment and portable device has been increasing. Such a biological certification using a fingerprint, a face, an iris, a palm pattern and the like derives an image of a living body from an image pick-up apparatus, performs a derivation of characteristics from the derived image, compares the information thus derived to the registered data to certify a person's identity.
Here, as a detection system of the image pick-up apparatus used in the biological certification system, there are available an optical system using an image pick-up device such as CCD, CMOS sensor and the like, an electric capacity system, a pressure detection system, a thermal sensitive system, an electric field detection system and the like. Further, as for another classification, there are available an area type for collectively deriving an object image by using a two dimensional area sensor, and an image pickup system called as a sweep type deriving a whole image by synthesizing the images of an object imaged in order in a sub-scanning direction by using a one dimensional line sensor or a band-shaped two dimensional sensor with the number of pixels being about five to twenty in a sub-scanning direction (for example, Japanese Patent Application Laid-Open No. 2002-216116).
Heretofore, in such a biological certification system, after having performed various image processings such as improvement of a contrast, an emphasis of edges and the like for the image derived by the image pick-up apparatus, a characteristic derivation processing for performing verification has been executed.
Further, a conventional art concerning an image input apparatus for performing an imaging by varying exposure conditions in different portions within a light receiving region has been disclosed (for example, see U.S. 2003 147550A1).
However, in the biological certification system, unless the original image itself has a sufficient image quality to a certain extent, a characteristic derivation level is downgraded, so that verification accuracy ends up being lowered. For example, in a fingerprint sensor of the optical system, a light source quality is an important factor to decide an image quality of the imaged image itself.
For example, in case a plurality of LED are turned into a light source, there arises a problem of non-uniformity of light volume of each light source (LED). For example, even if the LEDs are stratified according to a luminance rank, even in the case where there is a difference two times the same current value between the minimum value and the maximum value, such a difference is taken as within a tolerance level, and they are shipped out as the same products. That is, there exists large luminance unevenness for the same current value between the products.
In such a case, when a plurality of LEDs are taken as a light source, luminance unevenness appears as it is in a fingerprint image, and the maximum 50% of a dynamic range is occupied by non-uniformity of the light source, which bears heavily upon a signal component. This not only causes a lowering of a S/N but also makes the image processing for deriving characteristics of the imaged image such as background removal, ridge derivation and the like more difficult due to interference of the non-uniformity, and as a result, there arises a problem that the verification accuracy is lowered.
Further, particularly in the case of the optical type fingerprint sensor which induces a finger to adhere to the sensor, since the light source is arranged in the vicinity of the sensor, a light volume unevenness (shading) occurs, in which the light volume becomes uneven depending on a light receiving region on the sensor due to a distance relation between the light source and the sensor. In this case also, since the luminance difference appears as it is in an imaged fingerprint image, there arises a problem that the non-uniformity of the light source heavily bears upon a signal component as described above so as to lower the S/N and make the image processing such as background removal, ridge derivation, and the like difficult, thereby lowing the verification accuracy.
Particularly, in the case of the fingerprint sensor of a sweep type, there arise two peculiar problems as shown below due to an image quality deterioration of the imaged image. In the first place, there arises a problem of an image re-formation (processing for mutually joining partial images) becoming difficult. To re-form the image, after deriving the partial images of a finger (fingerprint) moving on the sensor, it is necessary to calculate a correlation between the partial images mutually neighbored up and down, and detect the same fingerprint region. At this time, when there exists a luminance difference due to a difference in the distance from the light source and a variation of the light source itself in the imaging region, the correlation is lowered by this much luminance difference, and therefore, there arises a problem that a detection of the same fingerprint region becomes difficult. This lowering of the correlation becomes remarkable particularly when the shading exists in a moving direction of the finger.
In the second place, there arises a problem that, when characteristic points of the fingerprint are derived from the image of the whole finger after the re-formation, erroneous characteristic points are detected or intrinsic characteristics are not detected, so that the verification accuracy ends up being lowered. In case there exists shading in a moving direction of the finger, a plurality of partial images having luminance difference are obtained in a column direction including a column having a high luminance (taken as a column which is read at first) and a column having a low luminance (taken as a column which is read at the last). When these partial images are to be continuously joined together, for example, a processing for joining the last column of the first partial image and the first column of the second partial image is performed. In this way, when an attempt is made to calculate the correlation between the last column (taken as a first column) of the first partial image and the first column (taken as a second column) of the second partial image and join the columns together, in case the luminance difference between the first column and the second column is large due to the shading, there arises a problem that a pseudo line is sometimes generated in the joined portion. There is also a problem that the same line appeared in the boundary for each partial image might be erroneously recognized as a pseudo line. In general, derivation of characteristic points of the fingerprint is performed by detecting the ridges of the fingerprint pattern, and discontinuous points and diverging points of the ridges are detected as characteristic points. Hence, if erroneously recognized as the pseudo fingerprint pattern, such discontinuous points and diverging points end up being generated, and the points which are not intrinsic characteristics are derived as the characteristic points. In this way, there arises a problem that the verification of the fingerprint is lowered.
Further, as a solution for these problems, though the correction of the derived imaged image by the image processing is considered, this correction often causes the deterioration of the S/N for the corrected portion or the side-effective deterioration of the image quality, and does not contribute to an essential solution.
Further, the optical sensor has a problem of being easily affected by an external light. In the case where the external light is weak inside the room or at night, and in the case where the object is impinged mainly by the light from a light source and the case where a strong incident light from the outside the room in the day time or a window impinges the object together with the light from the light source, the light volume and its distribution state sharply change, and the image derived ends up being sharply changed even if it is the same image.
For example, when it is optimized under a light volume condition at the in-door time, in case it is under a direct sunlight of midsummer, it does not enter a supposed dynamic range, but the image evaporates because of a strong light volume. On the contrary, when it is optimized under a bright environment, it turns brownish in case the light volume is too weak.
Further, in the case such as when the sunset light is incident obliquely from the window, and the like, when the light volume distribution within a plane sharply changes, a part of the image within the plane evaporates or turns brownish, so that an image range included in the dynamic range used for the image processing or the certification ends up being narrowed.
Because of the above described reasons, there arises a problem that the influence of the external light affects the computing time of the partial images, the computing result, the verification accuracy and the like.
Although a method for providing a light shielding cover to prevent the external light from entering the pixels of the sensor or providing an optical member for preventing the external light is considered, the light shielding cover and the optical member for preventing the external light invite an increase of enlargement of the external shape size and the cost thereof.
The present invention has been made in view of the above described circumstances, and solves at least one of the above described problems, and an object of the invention is to provide an image pick-up apparatus, a fingerprint certification apparatus and an image pick-up method, which can correct light volume unevenness due to variation of the light volume in a plurality of light sources.
Further, an object of the invention is to provide an image pick-up apparatus, a fingerprint certification apparatus and an image pick-up method, which can correct the shading generated due to a positional relation between the light source and the sensor.
Further, an object of the invention is to provide an image pick-up apparatus, a fingerprint certification apparatus and an image pick-up method, which detects and corrects the change and the non-uniformity of the light exposure due to the external light.